1. Field of the Invention
The invention relates to large pore structure crystalline aluminosilicate polymorphs, also known as zeolites, containing tetraethylammonium or methyltriethylammonium organic ions.
2. Description of the Prior Art
Large pore crystalline aluminosilicate zeolites having a silica to alumina ratio of at least four are desirable due to their catalytic selectivity and thermal stability. These properties are especially important for zeolites used as catalyst or in absorption processes under high temperatures.
Quaternary ammonium salts are used as templates or reaction modifiers to prepare synthetic zeolites that have a higher silica to alumina ratios than those found in naturally occurring zeolites. For example, U.S. Pat. No. 4,046,859 discloses a method for preparing a ferrierite structure zeolite (ZSM-21) that has a silica to alumina ratio greater than eight using a hydroxyethyl-trimethyl sodium aluminosilicate gel. Barre,, Zeolites, Vol. 1, p. 136 (1981) describes a method for obtaining zeolites using various ammonium organic bases as cations or reaction modifier salts to increase the silica to alumina ratio of the zeolite. Similarly, Lok et al., Zeolites, Vol. 3, p. 282 (1983) and Breck, Zeolite Molecular Sieves, pp. 348-349 (1974), provides a basic overview of zeolites synthesized with alkyl ammonium cations. Tetramethylammonium (TMA) cation containing salts are used as templates in the synthesis of zeolites A, Y and ZSM-4 (mazzite). See, for example, U.S. Pat. Nos. 3,306,992; 3,642,434; 4,241,036 and 3,923,639. However, TMA has a tendency to lodge in the small cavities of the zeolite's cage structure (sodalite or gemlinite cages), and is removed by burning the zeolite at high temperatures. This often leads to a disruption or total collapse of the zeolite cage structure. The the silica to alumina ratio in these structures is typically less than about 6.
Minor changes in the size or electron charge distribution of the organic cations used can induce the formation of different zeolite structures. For example, U.S. Pat. No. 4,046,859 shows that if one of the methyl groups of the TMA is replaced with a hydroxyethyl group, a secondary ferrierite-like phase (ZSM-21) will form in addition to the primary phase. Barrer, Zeolites, Vol. 1 (1981), describes other examples of how minor changes in cation distribution can induce the formation of various zeolite structures.
The theoretical structures possible, based on a series of interlinked truncated cubooctahedra (sodalite cages), are discussed in Moore and Smith, Mineralogical Magazine, Vol. 35, p. 1008-1014 (1964). The authors describe two types of zeolite structures comprised of connected sheets of linked sodalite cages. The first type has the sheet stacking sequence ABC ABC... (i.e., faujasite) while the second type has the sheet stacking sequence AB AB... and is referred to in the art as "Breck Structure 6" or "Breck 6". This structure is further described by Breck in Zeolite Molecular Sieves, p. 58 (1973) as being made of a hexagonal unit cell having the approximate cell dimensions of: a=17.5 angstroms and c=28.5 angstroms. "Breck 6" zeolites, containing the AB AB... stacking sequence, have been designated as ECR-30, see U.S. Pat. No. 4,879,103. In comparing the two structures, the faujasite is made of cubic packed (cp) sodalite cages while ECR-30 is made of hexagonally packed (hp) sodalite cages. Analogous structures of this type are found for carbon, i.e., diamond and lonsdaleite and for zinc sulfide, i.e., sphalerite and wurtzite.
Although, the two structures each contain the same connected sheets of linked sodalite cages, although stacked in different ways, each may randomly intergrow and produce different mixed structural composites known as intergrowths. Intergrowths are well known in zeolite mineralogy due to the increasing use of high resolution lattice imaging electron microscopy, such as that described by Millward et al. in Proc. Roy. Soc., A 399, p. 57 (1985) and by Rao and Thomas in Accounts of Chem. Res., Vol. 18, p. 113 (1985). For example, zeolite T is described in U.S. Pat. No. 2,850,952, and is intergrowth of erionite and offrette. (See, Bennett and Gard, Nature, Vol. 214, p. 1005 (1967) and U.S. Pat. No. 3,308,069.) Zeolite beta, has recently been characterized by Treacy and Newsam, Nature, Vol. 332, p. 249 (1988)) as an intergrowth of two enantiomorphs.
Various other modifications of the basic faujasite structure may be found in the literature. For example, U.S. Pat. No. 4,309,313, describes CSZ-1 as being made with cesium cations and having an X-ray diffraction pattern indexed on a hexagonal unit cell similar to that proposed for "Breck 6". However, it has recently been shown that CSZ-1 is made of a slightly distorted faujasite structure of twin planes near the center of very thin crystals. See Treacy et al., Jour. Chem. Soc. Chem. Commun., p. 1211 (1986). The twin planes create a strain in the faujasite lattice which causes a rhombohedral distortion in the cubic lattice structure of the faujasite. See Treacy et al, Analytical Electron Microscopy, San Francisco Press, p. 161-5, (1987). A faujasite crystal with individual double twin planes has also been observed by Thomas et al. reported in the Jour. Chem. Soc. Chem. Commun., (1981), p. 1221). Another faujasite-like material, ZSM-3, made with lithium and sodium, is described in U.S. Pat. No. 3,415,736. Although, having a hexagonal-like unit cell, similar to that of CSZ-1, the "c" axis of the diffraction plane for ZSM-3 could not be defined. See Kokotailo and Ciric, Molecular Sieve Zeolites-1. Amer. Chem. Soc. Adv. Chem. Ser. 101, Ed. Flanigen and Sand., p. 109 (1971). Therefore, ZSM-3 was believed to be made of a random stacking of faujasite and ECR-30 which means that the structure is a random mixture of the cp and hp forms. An infrared analysis of ZSM-3 later showed that it is more disordered than ZSM-20 described in U.S. Pat. No. 3,972,983. See Vaughan et al., Amer. Chem. Soc. Symp. Ser. 398, p. 544, (1989). A recent evaluation of ZSM-20 by Derouane et al., in Applied Catal., Vol. 28, p. 285, (1986) and by Ernst et al. in Zeolites, Vol. 7, p. 180 (1987) describes ZSM-20 as a faujasite-like material having spherical aggregates of twinned chunky crystals and a unit cell capable of being indexed on a hexagonal unit cell. Newsam et al., in Jour. Chem. Soc. Chem. Commun., p. 493, (1989) reported that ZSM-20 was an intimage intergrowth mixture of cp and hp layers with significant amounts of overgrown faujasite crystals.
An analysis of the known structural relationships between various faujasite and ECR-30 materials is summarized below:
TABLE 1 ______________________________________ Zeolite Si/Al Range Structure U.S. Pat. No. ______________________________________ X 1.0-1.5 cp U.S. Pat. No. 2,882,243 Y 1.5-3.0 cp U.S. Pat. No. 3,130,007 CSZ-3 1.5-3.5 cp U.S. Pat. No. 4,333,859 ECR-4 3.0-10.0 cp U.S. Pat. No. 4,965,059 CSZ-1 1.5-3.5 distorted cp U.S. Pat. No. 4,309,313 ZSM-3 1.4-2.25 random U.S. Pat. No. 3,415,736 cp/hp ZSM-20 3.5-infinity random U.S. Pat. No. 3,972,983 cp/hp ECR-30 3.0-10.0 hp U.S. Pat. No. 4,879,103 ECR-32 3.0-15.0 cp U.S. Pat. No. 4,931,267 ______________________________________
The morphology of ZSM-3 and ZSM-20, show that both crystals are about 0.6 micron in diameter and about 0.2 micron in thickness and that each has compressed octahedron shapes that are almost hexagonal and similar to the twinned "platelet faujasite" described in U.S. Pat. No. 4,175,059. On the other hand, ECR-30 and CSZ-1 both form thin plates up to about 1 micron in diameter and less than about 0. micron in thickness.
An object of the invention is to provide a intergrowth zeolite structure comprising blocks or zones (nano-crystals) of alternating faujasite and Breck 6 or ECR-30 structural units within the same macro-crystal, wherein the zeolite intergrowth may be characterized as a nano-moziac crystal of intergrown faujasite and ECR-30.
Another object of the invention is to provide zeolites having a basic faujasite sheet linked in a manner so as to form structures having large pores.
Another object of the present invention is to provide a method for producing large pore zeolites having unique structures and a ratio of silica to alumina of at least 4, wherein the organic templates used to formulate the zeolite does not lodge in the small cavities of the zeolite structure, but reside within the super cage structures of the zeolite and may be removed without disrupting or degrading the cage structure.
Other objects of the invention will become apparent to those skilled in the art upon reading the following description, to be taken in conjunction with the specific examples provided herein for illustrative purposes.